1. Field of the Invention
The present invention relates generally to medical systems and methods. More particularly, the present invention relates to the use of diffractive optics for generating successive patterns of light energy for ablating corneal or epithelial tissue.
Photorefractive keratectomy (PRK) and phototherapeutic keratectomy (PTK) employ optical beam delivery systems for directing laser energy to a patient""s eye in order to selectively ablate corneal tissue to reform the shape of the cornea and improve vision. All present commercial systems employ excimer lasers, where the beams from the lasers are spatially and temporally integrated in order to form a beam having uniform characteristics. In particular, the beams are integrated in order to display a flat intensity profile over a circular target region, often referred to as a xe2x80x9ctop hatxe2x80x9d profile.
Once such uniformly integrated beams are achieved, they may be used in different ways in order to effect corneal ablation. In a first type of system, the beam has a width which generally corresponds to the desired target area on the cornea. The beam intensity is manipulated using an iris or other exposure control mechanism, and the desired corneal reshaping can be achieved by properly controlling the exposure. While highly effective and relatively easy to control, the need to employ a laser beam having a width equal to the treatment area (typically on the order of 5.0 mm to 10.0 mm) requires the use of large excimer lasers. Not only are such large lasers expensive, they also occupy a relatively large area, requiring significant space to house them.
As an alternative to such large beam diameter systems, laser xe2x80x9cscanningxe2x80x9d systems are also employed for corneal ablation. Such scanning systems employ a much smaller beam width, minimizing energy required from the laser. The smaller lasers are both more economic and require less space to house them. The use of a small beam width, however, complicates certain aspects of the treatment protocols. As most of the small treatment beams have a circular diameter, it will be appreciated that it is difficult to control exposure of the cornea. In particular, the beams overlap in non-uniform patterns as they are scanned over the cornea, making it very difficult to achieve properly controlled exposure over the entire target region. While elaborate control and exposure algorithms have been developed to minimize detrimental variations in exposure, none are entirely adequate.
For these reasons, it would be desirable to provide improved methods and systems for the scanning of light beams over corneal tissue in order to selectively ablate the tissue to treat vision disorders. In particular, it would be desirable to utilize small beam geometries with low power requirements while achieving an even energy distribution free from small regions of overexposure and underexposure. Moreover, it would be desirable to simplify the control schemes and systems required to scan small width light beams for corneal treatment. It would further be desired to provide treatment protocols and algorithms which are particularly suitable for accommodating the circular geometry of an ablation zone on the cornea. It would be still further desirable if the methods and systems could be used for the ablation of the epithelial tissue over the cornea prior to corneal treatment. At least some of these objectives will be met by the invention described hereinafter.
2. Description of the Background Art
Scanning systems for performing photorefractive keratectomy (PRK) and phototherapeutic keratectomy (PTK) are described in a number of patents, including U.S. Pat. No. 5,391,165. A laser surgical system employing a diffractive optical element adapted to an individual patient is described in U.S. Pat. No. 5,571,107. A laser scanning system employing unique reflective optics is described in U.S. Pat. No. 5,546,214. A temporal and spatial beam integrated for a PRK/PTK laser system is described in U.S. Pat. No. 5,646,791. The full disclosures of each of the above-cited U.S. patents are incorporated herein by reference.
Use of a diffractive optical element for integrating an excimer laser beam for use in PRK/PTK procedures is described in a co-pending application entitled LASER SYSTEM AND METHOD WITH DIFFRACTIVE OPTIC, U.S. patent application Ser. No. 09/015,841, filed on Jan. 29, 1998, the full disclosure of which is incorporated herein by reference.
The present invention provides improved methods, systems, and other apparatus for performing scanning-type laser ablation.
The invention is particularly useful for performing corneal ablation in PRK and PTK procedures but will also be useful for removing an epithelial layer prior to corneal ablation in such procedures. For convenience, the following discussion will be directed at corneal ablation, but the teachings are also useful for removing epithelial tissue. The use of laser energy for removing epithelial tissue is described in co-pending U.S. patent application Ser. No. 09/022,774, filed on Feb. 12, 1998, the full disclosure of which is incorporated herein by reference.
By xe2x80x9cscanning,xe2x80x9d it is mean that an ablation light beam is aimed or xe2x80x9cscannedxe2x80x9d to successive, discrete locations on the corneal surface, and that those locations are then exposed to a predetermined amount or dosage of the light energy. Usually, the laser system will be operated in a pulsed manner, and the exposure at any particular location will result from a number of pulses which occur over a very short time period. The total area of the cornea to be treated, referred to hereinafter as the xe2x80x9cablation zone,xe2x80x9d is eventually treated as the ablative light beam scanned over the zone. As discussed above, however, prior systems which employ light beams having circular cross-sections result in an uneven treatment profile since adjacent circular geometries overlap in an uneven manner. The present invention significantly improves the uniformity of treatment, in some embodiments by employing beam geometries which are selected to cover the entire ablation zone without substantial overlap between adjacent beam patterns. In this way, the entire ablation zone can be treated with each segment or portion of the zone receiving the desired dosage of ablative energy.
The present invention provides a number of specific improvements over such prior corneal ablation methods and systems. First, the present invention provides methods and systems for treating an ablation zone with ablative light beams having annular or ring-shaped geometries. The ablation zone will usually have a circular geometry with a diameter in the range from 0.1 mm to 10.0 mm, usually from 1.0 mm to 6.0 mm. By employing successive ablative light beams having concentric, annular geometries, the entire ablation zone can be treated without substantial overlap between adjacent annular beams. That is, by utilizing adjacent annular light beams where the outer diameter of one beam is substantially equal to the inner diameter of the adjacent beam, each annular segment of the ablation zone will be treated only once. The use of annular beam geometries is particularly preferred since it facilitates dosage control over the ablation zone. That is, it will be relatively easy to expose the radially outward segments and radially inward segments to different energy dosages by properly selecting the beam intensity and dosage time of each of the radially positioned annular light beams. As described below, these annular beam geometries may also treat an ablation zone using substantial overlap between successive beams.
While the use of annular beam geometries will usually be preferred, the present invention could also utilize other beam geometries which are selected to cover individual treatment regions or segments over the total ablation zone without substantial overlap. In addition to the annular geometries described above, the treatment patterns could be pie-shaped, polygonal, irregular, or combinations thereof. In this aspect of the invention, it is important that the ablation zone be divided into a plurality of generally contiguous, non-overlapping patterns, and that each segment be treated with a discrete beam of ablative light. By then treating each of the contiguous segments with a desired dosage of the ablative light energy, total treatment of the ablation zone can be effected.
It will usually be preferably that each of the treatment segments of the ablation zone be treated only once, and that the treatment segments be non-overlapping. It would be possible, however, to devise annular, pie-shaped, or other segment geometries which are regularly and repeatedly overlapped to achieve the desired total dosage of the ablation zone in a uniform manner. For example, when employing annular treatment segments, successive treatment beams could be configured to overlap by a pre-selected distance, e.g., 50 percent of the width of the annulus. It would then be an easy matter to control the successive dosages so that each point within the ablation zone receives its desired dosage. Exposure at the boundary locations, i.e., the center region and outer peripheral annulus could be exposed with a half-width annulus or otherwise to achieve the desired exposure in those regions. Alternatively, the entire ablation zone can be treated with a first set of non-overlapping ablative light beams and thereafter treated with a second set of non-overlapping light beams, where the beams in the first and second set may overlap or non overlap. Thus, it can be seen that in some instances the present invention may rely on the use of successive overlapping light beams, where the beams overlap in a regular or predictable fashion which permit uniform treatment of the ablation zone to be achieved by properly controlling the energy distribution within the individual light beams.
In another aspect of the present invention, individual light beams having virtually any geometry or pattern may be provided using diffractive optics. In addition to providing the desired peripheral beam geometry, the diffractive optics may be used to provide a desired intensity profile within the individual light beam. Often, the intensity profile will be linear, i.e., will be flat or will uniformly rise or fall from the inner periphery of the light beam to the outer periphery. In other instances, the distribution may be gaussian, or may have other intensity profiles selected to achieve a desired result. Diffractive optics may be used to achieve the annular, pie-shaped, or other beam geometries described above. Often, a plurality of diffractive optical elements will be used to provide a corresponding plurality of beam geometries, where the beam geometries together cover the entire ablation zone in a non-overlapping or controlled overlapping fashion as described above. Alternatively, one or a limited number of diffractive optical elements may be utilized to establish a single beam geometry, and a beam expander used to expand or contract the beam to scan or cover the entire ablation zone. The use of a diffractive optical element together with a beam expander is particularly useful with annular beam geometries which can readily be expanded and decreased in diameter to provide successive, concentric beams to cover the entire circular ablation zone. Still further, a single limited number of diffractive optical elements can be used together with means for rotating the beam to scan the entire ablation zone. For example, when using pie-shaped beam geometries, the beam can be rotated to cover adjacent pie-shaped sections of the ablation zone in a non-overlapping or overlapping fashion. Rotation of the beam can be achieved by rotating the diffractive optical element or by providing optics downstream of the diffractive optical elements to rotate the beam in a selective fashion.
In all of the above, the ablative light energy will usually be laser energy, more usually being energy from an excimer laser, still more usually having a wavelength of about 193 nm. The beam patterns will typically have an area in the range from 0.01 mm2 to 50.0 mm2, more usually from 1.0 mm2 to 40.0 mm2. Successive beams may have the same or different areas, but each of the areas will typically be in the ranges set forth above. Each beam will typically have a power sufficient to maintain the minimum or threshold energy intensity required for corneal ablation, usually having a total energy in the range from 0.016 mJ to 80.0 mJ, usually from 1.6 mJ to 64.0mJ. Such energy levels can conveniently be achieved with excimer lasers having a total power output in the range from 0.16 mW to 80 W, usually from 1.0 mW to 10.0 W. The methods will usually employ at least about 3 successive patterns, usually at least about 30 successive patterns, typically being in the range from 4 to 1,500 successive patterns, more typically being in the range from 6 to 300 successive patterns.
The present invention also provides systems for ablating corneal tissue over an ablation zone. The systems comprise a coherent light source which produces an ablative light beam having a beam geometry. The system further comprises a means for transforming the beam into a plurality of successive patterns, where each pattern covers a portion of the ablation zone. Usually, each of the patterns will have a unique geometry, with no two patterns being the same. By xe2x80x9cunique geometry,xe2x80x9d it is meant that the beam will have a unique peripheral geometry, a unique orientation in space, and/or unique dimensions. Examples of such unique geometries, include annular rings having different diameters and usually different annular widths and pie-shaped patterns having different radial orientations but otherwise having similar dimensions. Usually, the different patterns will have a total area within the ranges set forth above. In the case of annular beam geometries, this will usually mean that the outermost rings will have smaller annular widths than the inner rings in order to help balance the beam intensity over the ablation zone. In some instances, of course, it may be desirable to have higher beam intensities near the center of the ablation zone when it is desired to remove more tissue in the center relative to the outer periphery. The systems will usually employ lasers or other coherent light sources having a power output in the range from 0.16 mW to 80.0 W.
A first exemplary system according to the present invention will employ a plurality of diffractive optical elements to produce the different beam patterns. In particular, each diffractive optical element will be adapted to transform the ablative light beam into one of the patterns. The system will further comprise a positioner which selectively places the diffractive optical elements in the path of the ablative light beam to transform the beam into the respective patterns. The diffractive optical elements will be adapted and selected so that the entire ablation zone can be covered by successively applying the light beams produced by the elements.
In a second exemplary system, means for transforming the ablative light beam will comprise one or a limited number of diffractive optical elements adapted to transform the ablative light beam into an annular pattern. The system will further comprise a beam expander which receives the ablative annular light beam and selectively adjusts the diameter of the annulus to produce the desired successive beam patterns.
The present invention still further comprises a programming card for use with a laser ablation system for ablating corneal tissue. The programming card comprises a tangible medium which stores computer-readable code. The code sets forth any of the methods set forth above for ablating corneal tissue.